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# Forces: Gravity, Friction, Constant

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19. An 80N crate slides with constant speed a distance of 5.0m downward along a rough slope that makes an angle of 30 degrees with the horizontal. The work done by the force of gravity is:
A. -400J
B. -200J
C. -69J
D. 200J
E. 400J

20. A man pulls a sled along a rough horizontal surface by applying a constant force F at an angle theta above the horizontal. In pulling the sled a horizontal distance d, the work done by the man is:
A. Fd
B. Fdcos(theta)
C. Fdsin(theta)
D. Fd/cos(theta)
E. Fd/sin(theta)

21. A man wishes to pull a crate 15 m across a rough floor by exerting a force of 100M. The coefficient of kinetic friction is 0.25. For the man to do the least work, the angle between the force and the horizontal should be:
A. 0
B. 14 degrees
C. 43 degrees
D. 66 degrees
E. 76 degrees

22. A particle moves 5m in the positive x direction while being acted upon by a constant force F = (4N)i + (2N)j - (4N)k. The work done on the particle by this force is:
A. 20 J
B. 10 J
C. -20J
D. 30J
E. is impossible to calculate without knowing other forces

23. A block is attached to the end of an ideal spring and moved from coordinate xi to coordinate xf. The relaxed position is at x = 0. The work done by spring is positive if:
A. xi = 2cm and xf = 4cm
B. xi = -2cm and xf = 4cm
C. xi = -2cm and xf = -4cm
D. xi = 2cm and xf = -4cm
E. xi = -4cm and xf = -2cm